Generally, a tractor is an engineering vehicle designed to deliver a high tractive force and conduct different kinds of work in the agriculture or construction field. At present, tractors for agriculture typically include a front loader provided on a front end of a tractor body, and an attachment mounted to the front loader so as to conduct various kinds of work, such as transportation, unloading, loading, etc. Furthermore, a rotavator or the like is mounted to a rear end of the tractor body by a rear connection link so that work such as plowing work can be conducted.
In such tractors for agriculture, power generated from an engine is classified into movement-related power which is transmitted to wheels through a transmission to enable a tractor to move, and PTO (power take-off)-related power which is used to drive different kinds of operating implements mounted to the tractor. To embody transmission of the movement-related power, a four-wheel drive system is typically used.
In the power system and structure of a typical four-wheel drive tractor, power generated from an engine such as a diesel engine is transmitted to a transmission through a clutch. The transmission conducts speed and torque conversion to provide controlled application of the power of the engine in response to movement conditions of the tractor and transmits the controlled power to a front wheel propulsion shaft and a rear propulsion shaft. The power transmitted to the front wheel propulsion shaft and the rear wheel propulsion shaft is transmitted to a front wheel differential device and a rear wheel differential device, respectively.
However, the power system of the conventional tractor is problematic in that, because the power of the engine is first sent to rear wheel axles and then transmitted from the rear wheel axles to front wheel axles, the rear wheel axles may forcibly propel the front wheel axles when the rear wheels of the rear wheel axles are not reduced in speed.
Typically, the conventional tractor is configured such that when a driver rotates a steering wheel in a clockwise or counterclockwise direction, the front wheels are turned to the left or the right by a steering apparatus. In the case where the front wheel axles are directly coupled to the respective front wheels, when the steering wheel is completely rotated, angles to which the front wheels can be turned cannot be over 60°. Therefore, although the steering wheel is completely rotated, the minimum turning radius of the tractor is comparatively wide. Thus, when the tractor is required to be turned around, the tractor must repeatedly move forwards and backwards.
The tractor having the above-mentioned construction must run or turn in place depending on conditions of a work site or the kind of work. As stated above, if the minimum turning radius of the tractor is comparatively wide, the tractor cannot cope with a small area or a case where it must turn around in place, thus reducing work efficiency.